cmake: link with --gc-sections

This linker flag, if supported, removes unused sections from the
executable.  Unused sections are those that do not contain public
symbols (therefore `-fvisibility=hidden`) and are not referenced by
other sections.

This shrinks the code size by 4%:

    text	  data	   bss	   dec	   hex	filename
 4069421	285904	1427713	5783038	583dfe	Debug/bin-native64/SphereSvrX64_release
 3918858	284456	1427713	5631027	55ec33	Debug/bin-native64/SphereSvrX64_release

cmake/CompilerFlagsBase.cmake

@@ -1,3 +1,4 @@
+include(CheckLinkerFlag)
 message(STATUS "Setting base compiler flags...")
 
 set(CMAKE_C_FLAGS           "${CMAKE_C_FLAGS} ${CUSTOM_C_FLAGS}")
@@ -56,6 +57,11 @@ if(NOT MSVC)
         set(local_link_options_nondebug)#-flto)
     endif()
 
+    check_linker_flag(CXX -Wl,--gc-sections LINKER_HAS_GC_SECTIONS)
+    if(LINKER_HAS_GC_SECTIONS)
+      list(APPEND local_link_options_nondebug -Wl,--gc-sections)
+    endif()
+
     set(custom_compile_options_release
         ${local_compile_options_nondebug}
         -flto=full